The technical field is fuel cells, with our without proton exchange membranes.
A fuel cell is an electrochemical apparatus wherein chemical energy generated from a combination of a fuel with an oxidant is converted to electric energy in the presence of a catalyst. The fuel is fed to an anode, which has a negative polarity, and the oxidant is fed to a cathode, which, conversely, has a positive polarity. The two electrodes are connected within the fuel cell by an electrolyte to transmit protons from the anode to the cathode. The electrolyte can be an acidic or an alkaline solution, or a solid polymer ion-exchange membrane characterized by a high ionic conductivity. The solid polymer electrolyte is often referred to as a proton exchange membrane (PEM).
In fuel cells employing liquid fuel, such as methanol, and an oxygen-containing oxidant, such as air or pure oxygen, the methanol is oxidized at an anode catalyst layer to produce protons and carbon dioxide. The protons migrate through the PEM from the anode to the cathode. At a cathode catalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations:
Anode reaction: CH3OH+H2Oxe2x86x926H++CO2+6exe2x88x92
Cathode reaction: 3/2 O2+6H++6exe2x88x92xe2x86x923H2O 
The essential requirements of typical fuel cells (see, e.g., FIG. 1) include: first, the fuel cell requires efficient delivery of fuel and air to the electrode, which typically requires complicated microchannels and plumbing structures. A second requirement is that the fuel cell should provide easy access to the catalyst and a large surface area for reaction. This second requirement can be satisfied by using an electrode made of an electrically conductive porous substrate that renders the electrode permeable to fluid reactants and products in the fuel cell. To increase the surface area for reaction, the catalyst can also be filled into or deposited onto a porous substrate. However, these modifications result in a fragile porous electrode that may need additional mechanical support, such as by use of a fiber matrix. Alternatively, the electrode can be made of an etched porous Vycor glass substrate or an etched-nuclear-particle-track membrane substrate to improve its toughness and strength. A third requirement is close contact between the electrode, the catalyst, and the PEM. The interface between the electrode and PEM is a discontinuity area as concerns the electric current transmission wherein the charge carriers are the electrons, on one side, and the protons on the other side. A solution to this problem has been attempted by hot pressing of the electrodes onto the PEM (U.S. Pat. No. 3,134,697). Another solution suggests the intimate contact of the catalytic particles with a protonic conductor before interfacing the electrode with the electrolyte (U.S. Pat. No. 4,876,115). Other solutions are described in U.S. Pat. Nos. 5,482,792 and 6,022,634. A fourth requirement is that the fuel cell should provide for humidity control of the electrode. The PEM requires water to be effective in conducting proton. However, since it operates at a higher temperature than its surroundings, the PEM tends to dehydrate during operation. The typical method of re-hydrating the PEM is to capture water in the exhaust stream and circulate it back to the PEM.
A flex based fuel cell comprises two flex circuits assembled face-to-face with PEM layers in between. Each flex circuit includes layers of a flex substrate, a patterned conductive material, a porous material sheet with a catalyst coating, and a PEM.
The catalyst coating on the porous material provides a large surface area for chemical reactions to proceed and small gaps in the porous material deliver liquid fuel to the catalyst through capillary force. The support of a soggy PEM on a porous electrode may be achieved by assembling two flex substrates face-to-face with the PEM constrained between the two flex substrates.
The flex substrates can bend in such a fashion to form a closed structure for confining the fuel. The delivery of the fuel can then be achieved by capillary force of a porous material sheet with the fuel distributed uniformly to all active surfaces, as long as a portion of the porous material sheet is in contact with the liquid fuel. Microchannels and plumbing are not required.
The flex based fuel cell can confine water between the two flex substrates to provide moisture for the PEM. Since deionized water can easily conduct protons but not electrons, a fuel cell can be constructed without the PEM. Two flex substrates without the PEM layer can be bonded together face-to-face with an adhesive layer in between as ridges.
The flex substrate, such as one using Kapton, provides additional advantages. The electrodes can be patterned directly on the flex substrate, thereby connecting different fuel cell panels in either serial or parallel manner.
In an embodiment, the flex substrates are formed into a cylinder. The interior side of the cylinder would be the fuel side, and the exterior of the cylinder would be the oxygen side. The fuel cell can be sealed at the bottom of the cylinder to provide a container for the liquid fuel. The liquid fuel, such as methanol, is delivered by the porous metal to the active catalytic surface of the interior side. Protons generated in the cylinder interior then diffuse through the PEM and reach the catalytic surface on the exterior side of the cylinder, where the protons combine with oxygen. The exterior side of the fuel cell is open to the atmosphere, which serves to supply the oxygen to the cylinder and carry away the reactant water vapor.
The flex substrates can be manufactured by the following steps:
(1) Patterning the flex substrate, such as Kapton (Dupont) or Upilex (Ube), with a thin film of conductive material. The patterning of the thin film provides the flexibility to define the size of fuel cell panels, as well as to configure the fuel cell for any particular current density or voltage output by routing the thin film patterning appropriately.
(2) Attaching a porous material sheet to the patterned thin film on the flex. In an embodiment, the porous material may be a porous metal. Alternatively, other porous materials may be used. For example, an organo-metallic solgel material may be attached to the patterned thin film. The attachment step can be accomplished by either attaching a commercially available porous metal sheet on the flex, or sintering a thick layer of porous metal on the flex. A thick porous metal layer can also be manufactured by applying a paste of low temperature metal powder, such as zinc nano particles (Aldrich catalog #48,393-1) mixed in glycol, and baking the resulting assembly in an oven to dry out the glycol and partially melt the metal particles. The metal powder paste can be screen printed on the flex substrate so that the location and shape of the sintered porous metal can conform to the thin film electrode previously patterned on the flex substrate.
As an alternative to the zinc powdered metal, silver powder may be used.
(3) Depositing a catalytic coating on the porous material sheet. Several catalytic materials, such as Ptxe2x80x94Ru and Ptxe2x80x94Ruxe2x80x94Os, are effective in converting methanol to proton without poisoning platinum in the flex substrate.
(4) Ablating backside openings to allow access to the catalytic surfaces. The flex substrate is ablated with a laser from the backside to create openings so that fuel on the cathode side and oxygen on the anode side can reach the active catalytic surfaces through the openings and the porous metal layer.
(5) For those flex substrates with a PEM, the surface of the catalytic coating may then be coated with a thin layer of PEM by dipping the structure into a 5% Nafion solution. A thin layer of PEM on top of the catalyst surface helps to capture protons. The thickness of the PEM may be controlled so that the liquid fuel can readily diffuse through the thin layer.
Two flex substrates can then be assembled face-to-face with the PEM in between to form a flex based fuel cell. For fuel cells without the PEM, two flex circuits manufactured with only the first four steps (without the PEM coating step) are assembled face-to-face with the catalytic coating layer in between.